Two lines of evidence from human and animal studies suggest that drugs activating kappa opioid receptors (KORs) may be uniquely analgesic in women. First, KOR agonists have recently been shown to relieve post- surgical pain better in women than in men, and KOR agonists are also more effective analgesics in female rats. Second, the well-documented increase in pain threshold during late pregnancy, i.e. the "analgesia of pregnancy", has been shown to involve KORs and their endogenous opioid ligand, dynorphin. These gender- and pregnancy-related variations in KOR responsiveness may be caused by differences in KOR distribution in pain-related neuronal circuits (e.g. in dorsal horn of the spinal cord, periaqueductal gray). The studies in this application are designed to test the hypotheses that (1) KORs are located on neurons critically involved in modulating pain transmission in the central nervous system (CNS); (2) the greater responsiveness of females to KOR agonists is correlated with higher levels of KORs in nociception-related regions of the CNS in females; and (3) increases in KORs in spinal cord occur during the endogenous opioid mediated "analgesia of pregnancy". The methods to be used include tract-tracing and single- and dual- labeling immunocytochemistry of brain and spinal cord, with quantitative light and electron microscopic analysis. Different aspects of these studies will be conducted in rats and guinea pigs. Rats have been better studied in behavioral nociception paradigms, and the results from experiments in rats will integrate well with a large literature on rat nociceptive systems. However, the distribution of KORs in guinea pig brain more closely resembles that of humans than does the rat, therefore the experiments in guinea pig may be more specifically related to the roles of KORs in humans. The results of these studies will be relevant to understanding gender differences in analgesia, and may aid in the development of analgesic drugs and their targeting to appropriate patient populations.